[mlir][tensor] Make getMixedPadImpl return static values when possible. (#85016)

If low and high are constants (i.e., not attributes), users still prefer
attributes. Otherwise, there could be failures in type inference. A
failure is introduced by
https://github.com/llvm/llvm-project/commit/60e562d11aeca8020de8d50ded7f0ba9e10e8843,
see the drop_known_unit_constant_low_high test for more details.